Hydromechanical steering transmissions for tracklaying or skid-steered vehicles, such as military tanks, are typically comprised of a range-changing gear pack and right and left hydrostatic drive units, all commonly driven in parallel by the vehicle prime mover operating at a substantially constant speed. Each hydrostatic unit includes a hydraulic pump and a hydraulic motor interconnected in a hydraulic fluid loop circuit. The hydraulic pump is driven by the mechanical output of the prime mover, such as a diesel or gas turbine engine, and, in turn, fluidically drives its hydraulic motor to produce a hydrostatic output. The speed of this hydrostatic output is determined by the relative hydraulic fluid displacements of the interconnected pump and motor. While it has been proposed that hydrostatic output speed can be varied by varying the displacements of both the pump and motor, in practice only the displacement of the pump is varied, while the motor is held at a constant, typically maximum displacement for optimum torque.
In hydromechanical steering transmissions of the type disclosed in Applicant's U.S. Pat. No. 4,345,488, the hydrostatic outputs of the two hydrostatic drive units are combined to produce a speed-averaged hydrostatic output which is applied as a second input to the range-changing gear pack along with the prime mover mechanical output. The range-changing gear pack thus produces a hydromechanical output which is combined with the hydrostatic output of the left hydrostatic unit in a left combining gear set and with the hydrostatic output of the right hydrostatic unit in a right combining gear set. The hydromechanical outputs of the left and right combining gear sets are produced on left and right transmission shafts, respectively, to drive the two vehicle tracks.
The range-changing gear pack is equipped with a plurality of brakes and clutches for selectively grounding and engaging gear elements thereof to incrementally change its input to output speed ratio and thus, to establish a plurality of forward speed ranges and at least one reverse speed range. Since the hydrostatic outputs are superimposed on the hydromechanical output of the range-changing gear pack by the combining gear sets, speed variation in each speed range is achieved by varying the hydrostatic output speeds, i.e., by varying the hydraulic pump displacements. This practice is commonly referred to as varying the "stroke" or "stroking" the hydraulic pumps. Since a hydraulic pump can be designed such that its displacement can be infinitely varied from 100% to zero displacement, hydrostatic output speed can be likewise infinitely varied between a maximum value and zero in either direction. Consequently the hydromechanical outputs of the transmission are capable of infinitely variable speeds within the limits of each speed range.
It will be appreciated that, when the hydraulic pumps of the left and right hydrostatic units are set at equal displacements or strokes the speeds of the left and right hydromechanical outputs of the transmission are equal, and straightline vehicle propulsion in the particular speed range established by the range-changing gear pack is produced. Uniform stroking of the two hydraulic pumps produces straightline acceleration or deceleration, depending upon stroking direction, of the vehicle. To steer the vehicle, the typical procedure is to uniformly stroke the two pumps in opposite directions, such that one transmission hydromechanical output is increased in speed to the same extent that the other transmission hydromechanical output is decreased in speed. The resulting speed differential imposed on the left and right tracks causes the vehicle to execute a steering maneuver.
Unfortunately, hydromechanical steering transmissions of this type are rather limited as to the top vehicle speed obtainable in the highest forward speed range. Certainly, the range-changing mechanism can be supplemented with a high, overdrive speed range, however this adds complexity, bulk and expense to the gear pack. In non-steering or so called "straight through" hydromechanical transmissions having one hydrostatic unit and one transmission output, it is common practice to stroke both the hydraulic pump and the hydraulic motor to extend the highest speed range and thus to increase top vehicle speed. However, in hydromechanical steering transmissions, wherein two hydrostatic units are jointly involved in both straightline and steer propulsion, hydraulic motor stroking of the hydraulic motors to increase top speed has not been implemented, primarily due to the potential adverse effects on output torque, steering performance, and the added complexities involved in the steering linkages and controls.
It is accordingly an object of the present invention to provide an improved hydromechanical steering transmission for tracklaying or skid-steered vehicles.
A further object is to provide a hydromechanical steering transmission of the above-character having improved maximum speed performance.
An additional object is to provide a method for controlling a hydromechanical steering transmission such as to achieve an extended top speed range and thus a higher vehicle top speed.
Another object is to provide a hydromechanical steering transmission of the above character, wherein a high top speed is achieved without prejudicing high speed steering performance.
Yet another object is to provide a hydromechanical steering transmission of the above-character, which is efficient in construction, convenient and simple to implement and control, and reliable in operation.
Other objects of the invention will in part be obvious and in part appear hereinafter.